Our objective is to increase our understanding of the mechanisms of implantation by studies of the changes in the surface coats of trophoblast cells, the processes by which trophoblast cells penetrate the uterine epithelium and maturation of blastomeres into trophoblast. Study of the surface coats will involve relationship to protein uptake and to adhesion, and will be examined by cytological, enzyme digestion, and agglutination methods. The mechanisms of implantation will be studied by combined scan and transmission electron microscopy, and by development of an in vitro implantation model. The aspects of differentiation of trophoblast that are being studied are the nature and fate of storage plaques, and formation and permeability of intertrophoblastic apical junctional complexes.